Solder joint structure and method for soldering electronic components

ABSTRACT

A method for soldering an electronic component is provided. A first solder land containing copper and a second solder land are formed on a surface of a circuit board. A first solder section composed of a Sn—Ag—Cu solder material is formed on each of the first and the second solder lands, and a terminal of an electronic component chip is mounted on the first solder land. The first solder land and the terminal are fusion-bonded. A second solder section composed of a Sn—Zn solder material is formed on the first solder section disposed on the second solder land. A lead terminal of another electronic component is inserted into a terminal hole formed near the second solder land; and the second solder section and the lead terminal are heated at a temperature lower than the temperature in step (d) to connect the lead terminal to the second solder section by fusion bonding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solder joint structure suitable foruse in lead-free soldering of an electronic component having a low heatresistance and to a method of soldering electronic components.

2. Description of the Related Art

In soldering terminals of various electronic components onto solderlands of print circuit boards, a Sn—Pb eutectic solder, e.g., 63Sn-37Pbsolder, has been widely used. As the use of lead-free solder materialsincreases to avoid environmental pollution, various proposals of soldermaterials that meet the demand for lead-free soldering are made. Amongsuch proposals, Sn—Ag—Cu solder materials are drawing much attentionsince they have superior thermal fatigue characteristics and creepproperty (e.g., Japanese Unexamined Patent Application Publication No.2002-158438). Whereas Sn—Pb eutectic solder melts at 183° C., themelting point of Sn—Ag—Cu solder materials is higher, i.e.,approximately 220° C. Accordingly, Sn—Ag—Cu solder materials are notpreferred in soldering electronic components having relatively low heatresistance. For example, there are a large number of lead-terminal-mountelectronic components that cannot withstand temperatures aboveapproximately 200° C. The terminals of such electronic components cannotbe soldered with Sn—Ag—Cu solder materials having the melting point ofapproximately 220° C.

Meanwhile, a tin-zinc (Sn—Zn) solder material has been known as alead-free solder material suitable for soldering thermolabile electroniccomponents (e.g., Japanese Unexamined Patent Application Publication No.2002-66783). The Sn—Zn solder material is an a prepared by adding 8percent by weight of zinc and 3 percent by weight of bismuth to tin,i.e., a Sn-8Zn-3Bi alloy. Since, the melting point of the alloy is low,i.e., approximately 200° C., the molten Sn—Zn solder can be joined withthe terminal of the thermolabile electronic component without inflictingany problem. However, a temperature cycling test in which the Sn—Znsolder material is applied on a patterned conductor containing copper,e.g., a copper foil, reveals that the joint strength decreases due tothe interaction between copper and zinc and that the reliability ofsoldering is seriously degraded as a result. In other words, requiredreliability cannot be achieved if soldering is performed by directlyapplying the Sn—Zn solder material on a solder land, i.e., a patternedcopper foil, on a print circuit board.

In order to overcome this problem, as shown in FIG. 7, a method wherebyunderlayers, namely, a nickel plating layer 3 and a thin gold platinglayer 4, are formed on a patterned copper foil 2 of a print circuitboard 1 before performing low-melting-point soldering on the goldplating layer 4 using a Sn—Zn solder 5 has been employed. The nickelplating layer 3 prevents zinc from diffusing into the patterned copperfoil 2. The gold plating layer 4 covers the nickel plating layer 3having poor solder wettability to secure joint with the Sn—Zn solder 5.A terminal hole 6 is formed in the patterned copper foil 2 on the printcircuit board 1. After the Sn—Zn solder 5 is applied on the gold platinglayer 4, a lead terminal 31 of a thermolabile electronic component isinserted into the terminal hole 6 and is heated in a reflow furnace atapproximately 200° C. so as to connect the lead terminal 31 to the Sn—Znsolder 5 by fusion bonding.

According to this solder joint structure in which the Sn—Zn solder 5 isformed on the patterned copper foil 2 with the nickel plating layer 3and the gold plating layer 4 therebetween, a decrease in joint strengthresulting from the interaction between copper and zinc can be avoided,thereby achieving reliable soldering of thermolabile electroniccomponents using lead-free solder materials.

The conventional solder joint structure shown in FIG. 7 simultaneouslyachieves reliable soldering of thermolabile electronic components whileavoiding environmental pollution resulting from use of lead and reliablesoldering of thermolabile electronic components. However, since thenickel plating layer 3, which has poor solder wettability, must becoated with the gold plating layer 4, material cost is high, and theprocess requires extra steps. Thus, increased cost has been a problem.

SUMMARY OF THE INVENTION

The present invention aims to overcome the problem experienced in theconventional art. A first object of the present invention is to providea lead-free solder joint structure that can achieve reliable solderingof thermolabile electronic components at low cost. A second object ofthe present invention is to provide a method for soldering electroniccomponents, whereby reliable lead-free soldering can be achieved at lowcost in mounting both heat-resistant electronic components andthermolabile electronic components onto a print circuit board.

In order to achieve the first object, the present invention provides asolder joint structure that includes a patterned conductor containingcopper; a solder base section composed of a Sn—Ag—Cu solder material ora Sn—Ag solder material; and a solder joint section composed of a Sn—Znsolder material. The solder joint section is disposed on the solder basesection, and the solder joint section connects with a terminal of anelectronic component by fusion bonding.

In this solder joint structure, when the solder base section is composedof the Sn—Ag—Cu solder material, the Sn—Ag—Cu solder material may befree of additives or may contain at least one additive selected fromantimony, nickel, phosphorus, germanium, and gallium. When the solderbase section is composed of the Sn—Ag solder material, the Sn—Ag soldermaterial preferably contains at least one additive selected fromantimony, nickel, phosphorus, germanium, gallium, aluminum, cobalt,chromium, iron, manganese, palladium, and titanium.

According to this solder joint structure in which the lead-free solderjoint section is formed on the patterned conductor, such as patternedcopper foil, containing copper with the lead-free solder base sectiontherebetween, a decrease in joint strength resulting from theinteraction between copper and zinc can be avoided due to the presenceof the solder base section. As a result, high reliability can beachieved without having to form expensive gold plating layers.Accordingly, reliable soldering of thermolabile electronic components bylow-melting-point Sn—Zn solder can be achieved at low cost.

In order to achieve the second object, the present invention provides amethod for soldering an electronic component, the method including (a)forming a first solder land, which is a patterned conductor, containingcopper and a second solder land, the first solder land and the secondsolder land being formed on the same surface of a circuit board; (b)forming a first solder section on each of the first solder land and thesecond solder land, the first solder section composed of a Sn—Ag—Cusolder material or Sn—Ag solder material; (c) mounting a terminal of anelectronic component chip on the first solder land; (d) heating thefirst solder land and the terminal to connect each other by fusionbonding; (e) forming a second solder section on the first solder sectiondisposed on the second solder land, the second solder section composedof a Sn—Zn solder material; (f) inserting a lead terminal of anotherelectronic component into a terminal hole formed near the second solderland; and (g) heating the second solder section and the lead terminal ata temperature lower than the temperature in step (d) so as to connectthe lead terminal to the second solder section by fusion bonding.

In this soldering method, when the first solder section is composed ofthe Sn—Ag—Cu solder material, the Sn—Ag—Cu solder material may be freeof additives or may contain at least one additive selected fromantimony, nickel, Phosphorus germanium, and gallium. When the firstsolder section is composed of the Sn—Ag solder material, the Sn—Agsolder material preferably contains at least one additive selected fromantimony, nickel, phosphorus, germanium, gallium, aluminum, cobalt,chromium, iron, manganese, palladium, and titanium.

According to this method, the first solder section is first formed onthe first solder land, and a terminal of a heat-resistant electroniccomponent chip is then mounted on the first solder land. When heated,highly reliable lead-free soldering can be performed. Soldering ofthermolabile electronic components must be performed after the solderingof the electronic component chip. In this method, the first soldersection is simultaneously formed on both the second solder lands and thefirst solder lands. Thus, the second solder section having a low meltingpoint can be formed on the first solder section, i.e., the underlayer,on the second solder land. Thus, reliable fusion bonding between thelead terminal of the thermolabile electronic component and the secondsolder section can be achieved. In particular, a decrease in jointstrength resulting from the interaction between copper and zinc can beprevented due to the presence of the first solder section, i.e., theunderlayer. Reliable soldering of the thermolabile electronic componentscan be performed using the lead-free Sn—Zn solder without having to formcostly gold plating layers. Moreover, no additional step of forming anunderlayer for the second solder section is necessary since the firstsolder section is already formed

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit board having various electronic componentsmounted thereon according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view of a solder joint structure for a leadterminal of a thermolabile electronic component shown in FIG. 1;

FIG. 3 shows lands on the circuit board;

FIG. 4 shows the lands on the circuit board, in which a Sn—Ag—Cu solderis applied on the lands by printing;

FIG. 5 shows the circuit board having electronic component chips mountedthereon;

FIG. 6 shows the circuit board having second solder lands for leadterminals, in which a Sn—Zn solder is applied on the second solder landsby printing; and

FIG. 7 is a cross-sectional view of a conventional solder jointstructure using a Sn—Zn solder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedwith reference to the drawings. FIG. 1 shows a circuit board havingvarious electronic components mounted thereon according to an embodimentof the present invention. FIG. 2 is a cross-sectional view of a solderjoint structure for a lead terminal of a thermolabile electroniccomponent. FIG. 3 shows lands on the circuit board. FIG. 4 shows thelands on the circuit board, which a Sn—Ag—Cu solder material is appliedon the lands by printing. FIG. 5 shows the circuit board havingelectronic component chips mounted thereon. FIG. 6 shows the circuitboard having second solder lands for lead terminals, in which a Sn—Znsolder material is applied on the solder lands by printing.

As shown in FIG. 3, first solder lands 11 for surface mounting andsecond solder lands 12 for use with lead terminals are formed on thesame surface of a circuit board 10. The first solder lands 11 arepatterned copper layers. A terminal hole 13 is formed near each secondsolder land 12. A terminal 21 of a heat-resistant electronic componentchip 20 will be mounted on each first solder land 11 and joined to thefirst solder land 11 by soldering. A terminal 31 of a thermolabileelectronic component 30 inserted into the terminal hole 13 will bejoined to the second solder land 12 by soldering.

In mounting the thermolabile electronic component 30 and theheat-resistant electronic component chip 20 onto the circuit board 10,the heat-resistant electronic component chip 20 is first mounted ontothe circuit board 10 by soldering at a relatively high temperature,e.g., approximately 230° C. to avoid heat damage on the thermolabileelectronic component 30. Subsequently, the thermolabile electroniccomponent 30 is mounted by soldering at a lower temperature, e.g.,approximately 200° C. In soldering the heat-resistant electroniccomponent chip 20 and the thermolabile electronic component 30,lead-free solder materials are used to avoid environmental pollution.

In particular, a first solder section 14 composed of a Sn—Ag—Cu soldermaterial is formed on each of the first solder lands 11 and the secondsolder lands 12 by printing, as shown in FIGS. 3 and 4. The soldermaterial is mainly composed of Sn and contains 3 percent by weight ofsilver (Ag) and 0.5 percent by weight of copper (Cu). The melting pointof the solder material is approximately 220° C.

Next, the terminal 21 of the heat-resistant electronic component chip 20is mounted on each first solder section 14 disposed on the first solderland 11 and is heated at approximately 230° C. in a reflow furnace. As aresult, the first solder section 14 connects with the terminal 21 byfusion bonding to end the process of mounting of the heat-resistantelectronic component chip 20, as shown in FIG. 5. As is previouslydescribed, the lead-free Sn—Ag—Cu solder material, which has superiorthermal fatigue characteristics and the like, securely solders theheat-resistant electronic component chip 20 on the first solder land 11by fusion bonding between the first solder section 14 and the terminal21.

Subsequently, as shown in FIG. 6, a second solder section 15 composed ofa Sn—Zn solder material is formed, by means of printing, on each of thefirst solder sections 14 disposed on the second solder lands 12. Thesolder material is mainly composed of tin (Sn) and contains 8 percent byweight of zinc and 3 percent by weight of bismuth (Sn-8Zn-3Bi). Themelting point is approximately 200° C. Bismuth is added because nosolder alloy composed of Sn and Zn only can be used in in-air soldering.

Next, the terminal 31 of the thermolabile electronic component 30 isinserted into each terminal hole 13 formed near the second solder land12 and is then heated at approximately 200° C. in a reflow furnace. As aresult, as shown in FIG. 2, the terminal 31 connects with the secondsolder section 15 by fusion bonding, thereby completing the mounting ofthe thermolabile electronic component 30. As is previously described,the joint strength between the patterned copper foil and the Sn—Znsolder material directly applied on the patterned copper foil becomesdegraded by the interaction between copper and zinc, the resultingjunction thereby failing to achieve the desired characteristics.However, in this embodiment, the first solder section 14 prevents adecrease in joint strength resulting from the interaction between copperand zinc. According to this embodiment, the thermolabile electroniccomponent 30 can be securely soldered on the second solder land 12 byusing a lead-free low-melting-point solder material, i.e., the secondsolder section 15, without having to use expensive gold plating layers.

In this embodiment, the first solder section 14 is also formed on eachof the second solder lands 12. The first solder section 14 disposed oneach second solder land 12 functions as the underlayer of the secondsolder section 15 that connects with the terminal 31 of the thermolabileelectronic component 30. According to this structure, the previouslymentioned problem of using the Sn—Zn solder material (the second soldersection 15), i.e., a decrease in joint strength due to the interactionbetween copper and zinc, is prevented. Thus, the thermolabile electroniccomponent 30 as well as the heat-resistant electronic component chip 20,can be reliably soldered by using the lead-free solder material. As aresult, expensive gold plating layers required for enhancing reliabilitybecome no longer necessary and substantial cost reduction can beachieved. Moreover, since the first solder sections 14 can besimultaneously formed on both the first solder land 11 and the secondsolder land 12, a separate step of forming an underlayer on the secondsolder land 12 is no longer necessary. The number of process steps canbe reduced.

In the above-described embodiment, the first solder sections 14 areformed on the first solder land 11 and the second solder land 12 byprinting. Alternatively, the first solder sections 14 may be formed onthe first solder land 11 and the second solder land 12 by dip soldering.

Furthermore, in the above-described embodiment, the first soldersections 14 are composed of a Sn—Ag—Cu solder material containing noadditives. The scope of the invention is not limited to this. Forexample, a Sn—Ag—Cu solder material containing a minute amount of atleast one of antimony, nickel, phosphorus, germanium, and gallium may beused. Moreover, the first solder sections 14 may be composed of a Sn—Agsolder material containing a minute amount of at least one of antimony,nickel, phosphorus, gerlmanium, gallium, aluminum, cobalt, chromium,iron, manganese, palladium, and titanium.

Preferably, the amount of the additive contained in the Sn—Ag—Cu soldermaterial or the Sn—Ag solder material is approximately 0.1 percent byweight. The melting point of the solder materials is preferablyapproximately 220° C. When the first solder sections 14 are composed ofthe Sn—Ag—Cu solder material containing antimony or nickel or the Sn—Agsolder material containing antimony or nickel, the thermal fatiguecharacteristics are further improved. When the first solder sections 14are composed of a Sn—Ag—Cu or Sn—Ag solder material containingphosphorus, germanium, gallium, or the like, reliability of solderingcan be improved since oxidation is inhibited.

1. A solder joint structure comprising: a patterned conductor containingcopper; a solder base section comprising a Sn—Ag—Cu solder material; anda solder joint section comprising a Sn—Zn solder material, the whereinthe solder joint section connects with a terminal of an electroniccomponent by fusion bonding.
 2. The solder joint structure according toclaim 1, wherein the Sn—Ag—Cu solder material contains at least oneadditive selected from the group consisting of antimony, nickel,phosphorus, germanium, and gallium.
 3. A solder joint structurecomprising: a patterned conductor containing copper; a solder basesection comprising a Sn—Ag solder material containing at least oneadditive selected from the group consisting of antimony, nickel,phosphorus, germanium, gallium, aluminum, cobalt, chromium, iron,manganese, palladium, and titanium; and a solder joint sectioncomprising a Sn—Zn solder material, the solder joint section beingdisposed on the solder base section, wherein the solder joint sectionconnects with a terminal of an electronic component by fusion bonding.4-6. (canceled)